In the prior art, industrial trucks are used for the placing into storage and removal from storage of loads in high rack warehouses. Examples of such industrial trucks are stacking vehicles such as forklift trucks and forklift reach trucks. A generic industrial truck comprises a lifting frame on which a load-receiving section is mounted so as to be movable vertically. The term “load-receiving section” is to be understood broadly in the present document and can comprise all components which are arranged so as to be movable vertically on the lifting frame. A load fork and a load fork carrier typically belong to the load-receiving section. In some generic industrial trucks, the load-receiving section further comprises a driver's station which is moved vertically on the lifting frame together with the load. Such industrial trucks are known under the term “man-up equipment” or vertical order picker.
It is a known problem that in generic industrial trucks, vibrations occur in the lifting frame. These vibrations are generally all the more pronounced, the further the load-receiving section is moved vertically, and the amplitude of the vibrations is greatest at the free end of the frame, i.e. in the region of the load-receiving section which has been moved upwards. As long as the load-receiving section oscillates to and fro due to the vibrations of the lifting frame, no loads can be received or deposited. Instead, the operator must wait with the stationary vehicle, until these vibrations have died down. Through these waiting times, however, the productivity of the vehicle is severely reduced. In man-up equipment, as a further problem it occurs that the oscillation movements of the driver's station, which has been moved upwards, are very unpleasant for the operator.
In DE 32 10 951 C2 the problems of lifting frame vibrations in the receiving and depositing of loads or respectively of load carriers, e.g. pallets, are described. According to the teaching of DE 32 10 951, reflex marks are applied to the shelf surfaces or to the pallets, said reflex marks being detected by a sensor fastened to the load fork, in order to determine the relative position of the shelf or pallet and load fork. By determining the relative position of shelf or pallet and load fork, the load fork can be aligned automatically for receiving or depositing the load. The actual function of the reflex mark and of the sensor consists in that the industrial truck can transport, receive and deposit loads automatically without a driver.
According to DE 32 10 951, however, with the aid of the reflex mark and the sensor in addition the phase and amplitude of the vibrations of the lifting frame can be measured in the direction of travel of the industrial truck. The load fork of the described industrial truck is adjustable relative to the lifting frame horizontally in the direction of travel and is provided with a corresponding drive means, in order to move the load fork forward and backward—in relation to the direction of travel of the industrial truck—on depositing or receiving loads. For the damping of the vibrations of the lifting frame in the plane of the direction of travel, DE 32 10 951 suggests actuating the drive means, which are present anyhow, between the load fork and the lifting frame so that the vibrations of the load fork are damped. For this, the drive means is regulated as a controlling element of a closed-loop control so that the sensor on the load fork is brought close to a desired zero position in relation to the reflex mark.
The described active vibration damping only takes place, however, on the receiving or respectively depositing of the load, because the entire controlling or respectively adjusting is oriented to the reflex mark on the shelf or on the pallet. The main purpose of DE 32 10 951 is therefore to shorten the waiting time of the stationary industrial truck in front of the shelf until the vibrations have died down sufficiently. However, this document does not give any suggestion of also already damping, during travel, vibrations, which typically are produced during travel.
A method which is similar in conceptual design for the damping of vibrations of the lifting frame of an industrial truck is known from applications DE 10 2007 015 488, DE 10 2007 024 817 A1 and EP 1 975 114 A1, which are related with regard to content and belong to the same patent family. In the industrial truck concerned, the lifting frame is able to be moved forward and back as a whole in the direction of travel. In order to damp vibrations of the lifting frame in the plane of the direction of travel, these applications suggest functionalizing the drive, which is present anyhow, for the horizontal displacement of the lifting frame in the direction of travel as a controlling element in a regulator circuit, with which vibrations of the lifting frame are to be regulated to zero. Herein, a sensor, by which the amplitude of the vibration of the lifting frame can be measured, serves as measurement indicator for the regulator, this sensor being in actual terms a strain gauge or an acceleration sensor on the lifting frame.
The prior art from the said patent family is similar to the above-mentioned DE 32 10 951 C2 in so far as a drive means, which is present in any case and is required for operating the industrial truck, is additionally used by suitable controlling or regulating in order to damp vibrations of the lifting frame. Differently from the DE 32 10 951 C2, however, according to the teaching of this patent family, vibrations can also be damped during travel.
Despite the steps for the damping of vibrations from the prior art described above, the problem of vibrations in industrial trucks can not be regarded as being solved. Especially with the higher travelling and working speeds which are aimed for in the course of increasing productivity, the vibrations on the lifting frame in practice often constitute a limiting factor.
The invention is therefore based on the problem of providing an industrial truck and a method in which vibrations in the lifting frame can be efficiently damped or even prevented entirely.